Machine to weed out dud PV wafers could save solar industry billions

Catches defective wafers before they're processed into solar cells.

The National Renewable Energy Laboratory has developed a means to screen dud silicon wafers prior to the manufacture of photovoltaic cells, a technique that the lab claims could save the solar industry billions of dollars.

A wafer is a fragile, 180-micrometer layer of semiconductor substrate which is doped with impurities to create the crucial p-n junctions that enable the flow of electrical current. The problem, according to the NREL, is that between 5 and 10 percent of wafers break during the PV cell manufacturing process. Failure is caused by minuscule cracks that occur as the wafers themselves are made, an intensive procedure that begins with raw silica, and involves furnaces, chemical purification, and multi-wire sawing, among other things.

The cracks leave some wafers too fragile to survive the expensive process of fabricating PV cells, in which the wafers are doped, fitted with electrical contacts, coated, and sealed. Even a process as simple as moving a wafer from one stage of cell manufacture to the next can prove fatal. The later the stage of failure, the more expensive it is to the manufacturer.

The NREL's Rene Rivero prepares a wafer for testing.

Dennis Schroeder

The machine heats individual strips of each wafer to about 500° C.

Dennis Schroeder

This wafer was one of the 5 to 10 percent which likely would have failed during PV cell manufacture

Dennis Schroeder

The NREL's idea is to weed out flawed wafers before they enter the PV cell manufacturing stage. To that end, it has created the Silicon Photovoltaic Wafer Screening System (leaving us with the less-than-compelling acronym SPWSS), a furnace designed to fit into existing factory assembly lines. The machine thermally stress-tests wafers by sequentially heating narrow strips of each to temperatures of about 500°C (the precise temperature can be calibrated to suit the needs of different manufacturers). NREL scientist Bhushan Sopori likens the process to filling a glass with very hot water. The idea is that, if the wafer is to break, it should break inside the new machine.

NREL is claimed that the system is extremely efficient, using "nearly 100 percent" of the input energy, keeping the cost of using the system down to what Sopori puts at "some fraction of a penny per wafer." (According to NREL, making solar cells costs approximately 15 cents per watt of capacity.) The lab estimates that by eliminating the 5 to 10 percent of cells with dud wafers, the solar industry could save billions of dollars. It hopes that this will be a tangible benefit to manufacturers in the US, where the share of the global PV market has fallen from 42 percent in 1997 to 4 percent in 2011.

The SPWSS will come in manual and automatic versions, both capable of processing 1,200 wafers per hour, which the NREL thinks is quick enough to keep pace with the production lines of most manufacturers. The basic SPWSS costs $60,000, but it doesn't automatically remove broken wafers; the automated $100,000 SPWSS-A does. Whether broken wafers are separated by machine or by hand, they can be recycled into new ones.

Automatic handling of lots of small thin shards isn't that easy. Especially if you don't want to damage other (intact) wafers on the same conveyor belt so you can't just vibrate it. Also, as roman said extra 40k$ isn't expensive for industrial machinery

$40,000 to detect that the wafer broke and then remove the broken wafer. Otherwise you need a person watching the line looking for broken wafers.

...yeah forgot the machine just breaks them.

so how do they detect the ones with non easily visible fractures? xray?

They probably crack all the way through, since identification of defective wafers by humans is one option. They're probably just using an off-the-shelf visual processing system tuned to look for cracked wafers. I'd be surprised if manufacturers would generally choose the automated option, though. Most will already have defect detection of various types incorporated into the manufacturing line and networked into a central QA database. Extending an existing system by adding a scanner right after the furnace would probably be more efficient/cheaper than trying to integrate NREL's detector.

Extending an existing system by adding a scanner right after the furnace would probably be more efficient/cheaper than trying to integrate NREL's detector.

The intent it to detect likely to fail wafers before they make into the normal manufacturing process; these later process steps are the expensive ones in terms of chemicals, metals, time, energy, etc.

Additionally this "detector" intentionally uses highly asymmetric heating to stress wafers so that bad ones break. This asymmetric heating should be far more energy efficient then later heating steps seen in typical furnaces. Also later heating steps in the process are design to be highly symmetric so they would not trigger similar failures.

Extending an existing system by adding a scanner right after the furnace would probably be more efficient/cheaper than trying to integrate NREL's detector.

The intent it to detect likely to fail wafers before they make into the normal manufacturing process; these later process steps are the expensive ones in terms of chemicals, metals, time, energy, etc.

Additionally this "detector" intentionally uses highly asymmetric heating to stress wafers so that bad ones break. This asymmetric heating should be far more energy efficient then later heating steps seen in typical furnaces. Also later heating steps in the process are design to be highly symmetric so they would not trigger similar failures.

If they're looking for cracks couldn't they just spray it with a special film and use a camera to detect fractures. They use the same process in making propellers for large ships minus the camera part. You wouldn't have to deal with broken pieces.

If they're looking for cracks couldn't they just spray it with a special film and use a camera to detect fractures. They use the same process in making propellers for large ships minus the camera part. You wouldn't have to deal with broken pieces.

The point is that they want to quickly and cheaply discard any wafer with hidden cracks.

There is no way to repair the cracks, and it is much, much quicker to heat fracture them than it is to use any other method to ID the flawed wafers as have been previously suggested.

For those not familiar with wafers, they are essentially ridiculously pure glass (silicon), and are cut thin enough that when they crack, they simply fall apart, unlike a wine glass that might remain intact after cracking due to the shape of the glass.

The broken pieces can then be melted and re-formed as new wafers and undergo the same test all over again...

Extending an existing system by adding a scanner right after the furnace would probably be more efficient/cheaper than trying to integrate NREL's detector.

The intent it to detect likely to fail wafers before they make into the normal manufacturing process; these later process steps are the expensive ones in terms of chemicals, metals, time, energy, etc.

Additionally this "detector" intentionally uses highly asymmetric heating to stress wafers so that bad ones break. This asymmetric heating should be far more energy efficient then later heating steps seen in typical furnaces. Also later heating steps in the process are design to be highly symmetric so they would not trigger similar failures.

He was talking about the $40k upgrade that detects failed wafers and handles them possibly being replaced by simple extensions to systems already in place, not replacing the entire machine.

You also don't want to spray any coating onto these virgin wafers because you'd have to strip it and get them atomically clean again before further processing. That would be expensive. This sounds like an excellent process. As mentioned, the machine cost is chump change. The finished PV wafers have, what, $150 or more added value in them? So every one that cracks now instead of near the end saves almost that muchmoney.

Cheaper alternative energy sources is always a plus in my book. How long do you think till the savings get passed down to the consumers?

Has that ever happened in real life?

But it is an interesting development and anything that reduces waste economically is a good thing.

Have you not seen the price of PV cells? Even without this the $/Watt at retail has been falling like a rock. It's a very competitive industry at the moment so any efficiency gains will end up largely with the consumer once more than one player can achieve them.

$40,000 to detect that the wafer broke and then remove the broken wafer. Otherwise you need a person watching the line looking for broken wafers.

...yeah forgot the machine just breaks them.

so how do they detect the ones with non easily visible fractures? xray?

They probably crack all the way through, since identification of defective wafers by humans is one option. They're probably just using an off-the-shelf visual processing system tuned to look for cracked wafers. I'd be surprised if manufacturers would generally choose the automated option, though. Most will already have defect detection of various types incorporated into the manufacturing line and networked into a central QA database. Extending an existing system by adding a scanner right after the furnace would probably be more efficient/cheaper than trying to integrate NREL's detector.

This machine does not detect defects, that is still done by the automated QA equipment. What it does do is stress the wafer before any of final processing is done. A very high percentage of the wafers that will not survive the manufacturing process will break when thermally stressed.

To see the effects of this test in your own kitchen.1) Place a thick water glass in the freezer (glass is silicon dioxide and has similar properties to the silicon wafers)2) Fill your tea kettle and set it on the stove to boil3) Put on a leather apron, leather gloves & a face shield (This is important as thermally stressed glass can fail catastrophically)4) Remove the glass from the freezer5) Pour a glass of boiling water6) Sweep up the remains

If they're looking for cracks couldn't they just spray it with a special film and use a camera to detect fractures. They use the same process in making propellers for large ships minus the camera part. You wouldn't have to deal with broken pieces.

That doesn't help with stressed parts just waiting for the final straw. This machine throws the whole bale of hay at such parts.

After it finishes, the wafer is sent to the type of machine you describe if it still appears to be intact.

Yes and yes. It's a novel idea that isn't obvious. The patent would be for the specific process. I.E. "Detection of defective silicon wafers by asymmetric heating" or something. This is a textbook case for what patents should be on, since it's a specific idea that isn't so broad as to cover the entire problem space.

$40,000 to detect that the wafer broke and then remove the broken wafer. Otherwise you need a person watching the line looking for broken wafers.

...yeah forgot the machine just breaks them.

so how do they detect the ones with non easily visible fractures? xray?

They probably crack all the way through, since identification of defective wafers by humans is one option. They're probably just using an off-the-shelf visual processing system tuned to look for cracked wafers. I'd be surprised if manufacturers would generally choose the automated option, though. Most will already have defect detection of various types incorporated into the manufacturing line and networked into a central QA database. Extending an existing system by adding a scanner right after the furnace would probably be more efficient/cheaper than trying to integrate NREL's detector.

This machine does not detect defects, that is still done by the automated QA equipment. What it does do is stress the wafer before any of final processing is done. A very high percentage of the wafers that will not survive the manufacturing process will break when thermally stressed.

To see the effects of this test in your own kitchen.1) Place a thick water glass in the freezer (glass is silicon dioxide and has similar properties to the silicon wafers)2) Fill your tea kettle and set it on the stove to boil3) Put on a leather apron, leather gloves & a face shield (This is important as thermally stressed glass can fail catastrophically)4) Remove the glass from the freezer5) Pour a glass of boiling water6) Sweep up the remains

Been there, done that. Several times actually. Guess it was my secret desire of "science"

This is very cool, except of course for the narrow bands where it is very hot. Kind of press-release-y, as someone already noted, but still a neat application of simple physics to solve a complex problem.

...it has created the Silicon Photovoltaic Wafer Screening System (leaving us with the less-than-compelling acronym SPWSS)...

Technically, that is an initialism, not an acronym.

Also, the notion that a heat-generating machine is efficient seems highly dubious. Are they saying that nearly 100% of the input energy is transferred to the wafer as heat?

Also, as others have noted, $60,000-100,000 is cheap for industrial mass production equipment, and this particular machine may well have very high throughput. I did not realize that sorting polycrystalline silicon wafers for structural defects was such a problem in the PV industry. This machine will probably quickly become as standard as furnaces. The asymmetrical heating probably limits its use to pre-doped wafers as the uneven heating will also cause uneven diffusion of the dopants (maybe not too great an effect at 500 degrees C). If wafer breakage if primarily due to structural defects present in pre-processed wafers, this could be a huge help.

$40,000 to detect that the wafer broke and then remove the broken wafer. Otherwise you need a person watching the line looking for broken wafers.

...yeah forgot the machine just breaks them.

so how do they detect the ones with non easily visible fractures? xray?

They probably crack all the way through, since identification of defective wafers by humans is one option. They're probably just using an off-the-shelf visual processing system tuned to look for cracked wafers. I'd be surprised if manufacturers would generally choose the automated option, though. Most will already have defect detection of various types incorporated into the manufacturing line and networked into a central QA database. Extending an existing system by adding a scanner right after the furnace would probably be more efficient/cheaper than trying to integrate NREL's detector.

This machine does not detect defects, that is still done by the automated QA equipment. What it does do is stress the wafer before any of final processing is done. A very high percentage of the wafers that will not survive the manufacturing process will break when thermally stressed.

To see the effects of this test in your own kitchen.1) Place a thick water glass in the freezer (glass is silicon dioxide and has similar properties to the silicon wafers)2) Fill your tea kettle and set it on the stove to boil3) Put on a leather apron, leather gloves & a face shield (This is important as thermally stressed glass can fail catastrophically)4) Remove the glass from the freezer5) Pour a glass of boiling water6) Sweep up the remains

The whole discussion you are responding to is about the "Automated" version of the machine which does also have the automated QA equipment incorporated into the purchase price.

I smell hype. There's already a large industry for wafer equipment, and the difference between a prototype and volume production (of the machines, not the wafers) is huge. It sounds to me like this (government funded??) organization is trying to get some PR. The article seems to be 100% based on the claims in a press release, which is not a good sign.

Tune back in 3 years and we'll see if this system has made it to the market. I hope I'm wrong.

Extending an existing system by adding a scanner right after the furnace would probably be more efficient/cheaper than trying to integrate NREL's detector.

The intent it to detect likely to fail wafers before they make into the normal manufacturing process; these later process steps are the expensive ones in terms of chemicals, metals, time, energy, etc.

Additionally this "detector" intentionally uses highly asymmetric heating to stress wafers so that bad ones break. This asymmetric heating should be far more energy efficient then later heating steps seen in typical furnaces. Also later heating steps in the process are design to be highly symmetric so they would not trigger similar failures.

He was talking about the $40k upgrade that detects failed wafers and handles them possibly being replaced by simple extensions to systems already in place, not replacing the entire machine.

This machine tests nothing. It stresses the wafer. The $40,000 automation package removes the shattered wafers. There is no further checking done by this deviceA clearer statement of what this tester does is in the original press release.http://www.nrel.gov/news/features/featu ... re_id=2076

Conventional testing that looks for cracking not noticed by the furnace operator will still catch a few pieces, but the majority of the failed wafers will shatter in this furnace.